arXiv subjects

Amr El-Zant

Publications and source records attributed to Amr El-Zant.

At least 19 recordsLinked to original sources

arXiv2025

Clustered unified dark sector cosmology: Background evolution and linear perturbations in light of observations

We consider unified dark sector models in which the fluid can collapse and cluster into halos, allowing for hierarchical structure formation to proceed as in standard cosmology. We show that both background evolution and linear perturbations tend towards those in $\LCDM$ as the clustered fraction $f \rightarrow 1$. We confront such models with various observational datasets, with emphasis on the relatively well motivated standard Chaplygin gas. We show that the strongest constraints come from secondary anisotropies in the CMB spectrum, which prefer models with $f \rightarrow 1$. However, as a larger Hubble constant is allowed for smaller $f$, values of $f \simeq 0.99$ (rather than tending to exact unity) are favored when late universe expansion data is included, with $f \simeq 0.97$ and $H_0 \simeq 70 {\rm km/s/Mpc}$ allowed at the 2-$σ$ level. Such values of $f$ imply extremely efficient clustering into nonlinear structures. They may nevertheless be compatible with clustered fractions in warm dark matter based cosmologies, which have similar minimal halo mass scales as the models considered here. Tight CMB constraints on $f$ also apply to the generalized Chaplygin gas, except for models that are already quite close to $\LCDM$, in which case all values of $0 \le f \le 1$ are allowed. In contrast to the CMB, large scale structure data, which were initially used to rule out unclustered unified dark matter models, are far less constraining. Indeed, late universe data, including the large scale galaxy distribution, prefer models that are far from $\LCDM$. But these are in tension with the CMB data.

astro-ph.CO↗

arXiv2024

Building stellar bulges and halo cores from massive clumps observed in the DYNAMO-HST sample

We present N-body simulations of the process of bulge formation in disc galaxies due to inward migration of massive stellar clumps. The process is accompanied by dark halo heating, with a quasi-isothermal core replacing the initial central density cusp, transforming an initially dark matter dominated central region into a baryon dominated one. The characteristics of the clumps are chosen to be compatible with low redshift observations of stellar clumps in DYNAMO-HST galaxies, which may be relatively long lived in terms of being robust against internal starburst-instigated disruption. We thus test for disruption due to tidal stripping using different clump internal radial profiles; Plummer, Hernquist and Jaffe, in ascending order of steeper central density profile. Our calculations predict that in order for clump migration to be effective in building galactic bulges and dark halo cores, steeply increasing central clump profiles, or a less massive or less concentrated haloes, are preferred. The dependence on such factors may contribute to the diversity in observed total mass distributions and resulting rotation curves in galaxies. When the process is most efficient, a 'bulge-halo conspiracy', with a singular isothermal total density akin to that observed bright galaxies, results.

astro-ph.GA↗

arXiv2023

Halo heating from fluctuating gas in a model dwarf

The cold dark matter (CDM) structure formation scenario faces challenges on (sub)galactic scales, central among them being the `cusp-core' problem. A known remedy, driving CDM out of galactic centres, invokes interactions with baryons, through fluctuations in the gravitational potential arising from feedback or orbiting clumps of gas or stars. Here we interpret core formation in a hydrodynamic simulation in terms of a theoretical formulation, which may be considered a generalisation of Chandrasekhar's theory of two body relaxation to the case when the density fluctuations do not arise from white noise; it presents a simple characterisation of the effects of complex hydrodynamics and `subgrid physics'. The power spectrum of gaseous fluctuations is found to follow a power law over a range of scales, appropriate for a fully turbulent compressible medium. The potential fluctuations leading to core formation are nearly normally distributed, which allows for the energy transfer leading to core formation to be described as a standard diffusion process, initially increasing the velocity dispersion of test particles as in Chandrasekhar's theory. We calculate the energy transfer from the fluctuating gas to the halo and find it consistent with theoretical expectations. We also examine how the initial kinetic energy input to halo particles is redistributed to form a core. The temporal mass decrease inside the forming core may be fit by an exponential form; a simple prescription based on our model associates the characteristic timescale with an energy relaxation time. We compare the resulting theoretical density distribution with that in the simulation.

astro-ph.GA↗

arXiv2022

The growth of fluctuations in Chaplygin gas cosmologies: A nonlinear Jeans scale for unified dark matter

Unified dark matter cosmologies economically combine missing matter and energy in a single fluid. Of these models, the standard Chaplygin gas is theoretically motivated, but faces problems in explaining large scale structure if linear perturbations are directly imposed on the homogeneous fluid. However, early formation of a clustered component of small halos is sufficient (and necessary) for hierarchical clustering to proceed in a CDM-like component as in the standard scenario, with the remaining homogeneous component acting as dark energy. We examine this possibility. A linear analysis shows that a critical Press-Schecter threshold for collapse can generally only be reached for generalized Chaplygin gas models that mimic $Λ$CDM, or ones where superluminal sound speeds occur. But the standard Chaplygin gas case turns out to be marginal, with overdensities reaching order one in the linear regime. This motivates a nonlinear analysis. A simple infall model suggests that collapse is indeed possible for perturbations of order 1~kpc and above; for, as opposed to standard gases, pressure forces decrease with increasing densities, allowing for the collapse of linearly stable systems. This suggests that a cosmological scenario based on the standard Chaplygin gas may not be ruled out from the viewpoint of structure formation, as often assumed. On the other hand, a 'nonlinear Jeans scale', constricting growth to scales $R \gtrsim {\rm kpc}$, which may be relevant to the small scale problems of CDM, is predicted. Finally, the background dynamics of clustered Chaplygin gas cosmologies is examined and confronted with observational datasets. It is found to be viable (at 1-sigma), with a mildly larger $H_0$ than $Λ$CDM, if the clustered fraction is larger than $90 \%$.

astro-ph.CO↗

arXiv2022

Facilitating Non-HEP Career Transition

About two-third of Physics PhDs establish careers outside of academia and the national laboratories in areas like Software, Instrumentation, Data Science, Finance, Healthcare, Journalism, Public Policy and Non-Governmental Organization. Skills and knowledge developed during HEPA (High Energy Physics and Astrophysics) research as an undergraduate, graduate or a postdoc level (collectively called early career) have been long sought after in industry. These skills are complex problem solving abilities, software programming, data analysis, math, statistics and scientific writing, to name a few. Given that a vast majority transition to the industry jobs, existing paths for such transition should be strengthened and new ways of facilitating it be identified and developed. A strong engagement between HEPA and its alumni would be a pre-requisite for this. It might also lead to creative ways to reverse the "brain drain" by encouraging alumni to collaborate on HEPA research projects or possibly come back full time to research. We motivate and discuss below several actionable recommendations by which HEPA institutions as well as HEPA faculty mentors can strengthen both ability to identify non-HEP career opportunities for students and post-docs as well as help more fully develop skills such as effective networking, resume building, project management, risk assessment, budget planning, to name a few. This will help prepare early career HEPA scientists for successfully transitioning from academia to the diverse array of non-traditional careers available. HEPA alumni can play a pivotal role by engaging in this process.

physics.ed-ph↗

arXiv2020

Ejection of supermassive black holes and implications for merger rates in fuzzy dark matter haloes

Fuzzy dark matter (FDM) consisting of ultra-light axions has been invoked to alleviate galactic-scale problems in the cold dark matter scenario. FDM fluctuations, created via the superposition of waves, can impact the motion of a central supermassive black hole (SMBH) immersed in an FDM halo. The SMBH will undergo a random walk, induced by FDM fluctuations, that can result in its ejection from the central region. This effect is strongest in dwarf galaxies, accounting for wandering SMBHs and the low detection rate of AGN in dwarf spheroidal galaxies. In addition, a lower bound on the allowed axion masses is inferred both for Sagittarius $A^*$ and heavier SMBH; to avoid ejection from the galactic centres, axion masses of the order of $10^{-22}{\rm eV}$ or lighter are excluded. Stronger limits are inferred for merging galaxies. We find that the event rate of SMBH mergers in FDM haloes and the associated SMBH growth rates can be reduced by at least an order of magnitude.

astro-ph.GA↗

arXiv2019

The effect of fluctuating fuzzy axion haloes on stellar dynamics: a stochastic model

Fuzzy dark matter of ultra-light axions has gained attention, largely in light of the galactic scale problems associated with cold dark matter. But the large de Broglie wavelength, believed to possibly alleviate these problems, also leads to fluctuations that place constraints on ultra-light axions. We adapt and extend a method, previously devised to describe the effect of gaseous fluctuations on cold dark matter cusps, in order to determine the imprints of ultra-light axion haloes on the motion of classical test particles. We first evaluate the effect of fluctuations in a statistically homogeneous medium of classical particles, then in a similar system of ultra light axions. In the first case, one recovers the classical two body relaxation time (and diffusion coefficients) from white noise density fluctuations. In the second situation, the fluctuations are not born of discreteness noise but from the finite de Broglie wavelength; correlation therefore exists over this scale, while white noise is retained on larger scales, elucidating the correspondence with classical relaxation. The resulting density power spectra and correlation functions are compared with those inferred from numerical simulations, and the relaxation time arising from the associated potential fluctuations is evaluated. We then apply our results to estimate the heating of disks embedded in axion dark haloes. We find that this implies an axion mass $m \ga 2 \times 10^{-22} {\rm eV}$. We finally apply our model to the case of the central cluster of Eridanus II, confirming that far stronger constraints on $m$ may in principle be obtained, and discussing the limitations associated with the assumptions leading to these.

astro-ph.GA↗

arXiv2019

Errors, chaos and the collisionless limit

We simultaneously study the dynamics of the growth of errors and the question of the faithfulness of simulations of $N$-body systems. The errors are quantified through the numerical reversibility of small-$N$ spherical systems, and by comparing fixed-timestep runs with different stepsizes. The errors add randomly, before exponential divergence sets in, with exponentiation rate virtually independent of $N$, but scale saturating as $\sim 1/\sqrt{N}$, in line with theoretical estimates presented. In a third phase, the growth rate is initially driven by multiplicative enhancement of errors, as in the exponential stage. It is then qualitatively different for the phase space variables and mean field conserved quantities (energy and momentum); for the former, the errors grow systematically through phase mixing, for the latter they grow diffusively. For energy, the $N$-variation of the `relaxation time' of error growth follows the $N$-scaling of two-body relaxation. This is also true for angular momentum in the fixed stepsize runs, although the associated error threshold is higher and the relaxation time smaller. Due to shrinking saturation scales, the information loss associated with the exponential instability decreases with $N$ and the dynamical entropy vanishes at any finite resolution as $N \rightarrow \infty$. A distribution function depending on the integrals of motion in the smooth potential is decreasingly affected. In this sense there is convergence to the collisionless limit, despite the persistence of exponential instability on infinitesimal scales. Nevertheless, the slow $N$-variation in its saturation points to the slowness of the convergence.

astro-ph.IM↗

arXiv2018

$H_0$ Tension and the Phantom Regime: A Case Study In Terms of an Infrared $f(T)$ Gravity

We propose an $f(T)$ teleparallel gravity theory including a torsional infrared (IR) correction. We show that the governing Friedmann's equations of a spatially flat universe include a phantom-like effective dark energy term sourced by the torsion IR correction. As has been suggested, this phantom phase does indeed act as to reconcile the tension between local and global measurements of the current Hubble value $H_0$. The resulting cosmological model predicts an electron scattering optical depth $τ_e\thickapprox 0.058$ at reionization redshift $z_{re} \sim 8.1$, in agreement with observations. The predictions are however in contradiction with baryon acoustic oscillations (BAO) measurements, particularly the distance indicators. We argue that this is the case with any model with a phantom dark energy model that has effects significant enough at redshifts $z \lesssim 2$ as to be currently observable. The reason being that such a scenario introduces systematic differences in terms of distance estimates in relation to the standard model; e.g., if the angular diameter distance to the recombination era is to be kept constant while $H_0$ is increased in the context of a phantom scenario, the distances there are systematically overestimated to all objects at redshifts smaller than recombination. But no such discrepancies exist between $Λ$CDM predictions and current data for $z \lesssim 2$.

gr-qc↗

arXiv2016

From cusps to cores: a stochastic model

The cold dark matter model of structure formation faces apparent problems on galactic scales. Several threads point to excessive halo concentration, including central densities that rise too steeply with decreasing radius. Yet, random fluctuations in the gaseous component can 'heat' the centres of haloes, decreasing their densities. We present a theoretical model deriving this effect from first principles: stochastic variations in the gas density are converted into potential fluctuations that act on the dark matter; the associated force correlation function is calculated and the corresponding stochastic equation solved. Assuming a power law spectrum of fluctuations with maximal and minimal cutoff scales, we derive the velocity dispersion imparted to the halo particles and the relevant relaxation time. We further perform numerical simulations, with fluctuations realised as a Gaussian random field, which confirm the formation of a core within a timescale comparable to that derived analytically. Non-radial collective modes enhance the energy transport process that erases the cusp, though the parametrisations of the analytical model persist. In our model, the dominant contribution to the dynamical coupling driving the cusp-core transformation comes from the largest scale fluctuations. Yet, the efficiency of the transformation is independent of the value of the largest scale and depends weakly (linearly) on the power law exponent; it effectively depends on two parameters: the gas mass fraction and the normalisation of the power spectrum. This suggests that cusp-core transformations observed in hydrodynamic simulations of galaxy formation may be understood and parametrised in simple terms, the physical and numerical complexities of the various implementations notwithstanding.

astro-ph.GA↗

arXiv2015

Unified dark matter: constraints from galaxies and clusters

Unified dark matter models are appealing in that they describe the dark sector in terms of a single component. They however face problems when attempting to account for structure formation: in the linear regime, density fluctuations can become Jeans stable and oscillate rather than collapse, though it is possible that this difficulty may be circumvented by invoking nonlinear clustering. Here we examine the behaviour in the fully nonlinear regime, of collapsed objects that should mimic standard dark matter haloes. It is shown that the pressure gradient associated with the unified dark matter fluid should be significant in the outer parts of galaxies and clusters, and its effects obervable. In this case, no flat or falling rotation curve is possible for any (barotropic) equation of state with associated sound speed decreasing with density (a necessary condition if the fluid is to behave as pressureless matter at high density). The associated density profile is therefore also incompatible with that inferred in the outer part of clusters. For the prototypical case of the generalised Chaplygin gas, it is shown that this limits the values of the equation of state index $α$ that are compatible with observations to $α\la 0.0001$ or $α\ga 2$. This is in line from what is deduced from linear analysis. More generally, from the expected properties of dark matter haloes, constraints on the sound speed are derived. For the particular case of the generalised Chaplygin gas, this further constrains the index to $α\la 10^{-9}$ or $α\ga 6.7$. For a unified dark matter fluid to mimic dark halo properties, therefore, it needs to have an equation of state such that the pressure gradients are either minimal or which decrease fast enough so as to be negligible at densities characteristic of the outer parts of haloes.

astro-ph.CO↗

arXiv2015

Warm Dark Matter in B-L Inverse Seesaw

We show that a standard model gauge singlet fermion field, with mass of order keV or larger, and involved in the inverse seesaw mechanism of light neutrino mass generation, can be a good warm dark matter candidate. Our framework is based on B-L extension of the Standard Model. The construction ensures the absence of any mixing between active neutrinos and the aforementioned dark matter field. This circumvents the usual constraints on the mass of warm dark matter imposed by X-ray observations. We show that over-abundance of thermally produced warm dark matter (which nevertheless do not reach chemical equilibrium) can be reduced to an acceptable range in the presence of a moduli field decaying into radiation --- though only when the reheat temperature is low enough. Our warm dark matter candidate can also be produced directly from the decay of the moduli field during reheating. In this case, obtaining the right amount of relic abundance, while keeping the reheat temperature high enough as to be consistent with Big Bang nucleosynthesis bounds, places constraints on the branching ratio for the decay of the moduli field into dark matter.

hep-ph↗

arXiv2013

Dark halos and elliptical galaxies as marginally stable dynamical systems

The origin of equilibrium gravitational configurations is sought in terms of the stability of their trajectories, as described by the curvature of their Lagrangian configuration manifold of particle positions --- a context in which subtle spurious effects originating from the singularity in the two body potential become particularly clear. We focus on the case of spherical systems, which support only regular orbits in the collisionless limit, despite the persistence of local exponential instability of $N$-body trajectories in the anomalous case of discrete point particle representation even as $N \rightarrow \infty$. When the singularity in the potential is removed, this apparent contradiction disappears. In the absence of fluctuations, equilibrium configurations generally correspond to positive scalar curvature, and thus support stable trajectories. A null scalar curvature is associated with an effective, averaged, equation of state describing dynamically relaxed equilibria with marginally stable trajectories. The associated configurations are quite similar to those of observed elliptical galaxies and simulated cosmological halos, and are necessarily different from the systems dominated by isothermal cores, expected from entropy maximization in the context of the standard theory of violent relaxation. It is suggested that this is the case because a system starting far from equilibrium does not reach a 'most probable state' via violent relaxation, but that this process comes to an end as the system finds and (settles in) a configuration where it can most efficiently wash out perturbations. We explicitly test this interpretation by means of direct simulations.

astro-ph.GA↗

arXiv2012

The Geometrodynamical Origin of Equilibrium Gravitational Configurations

The origin of equilibrium gravitational configurations is sought in terms of the stability of their trajectories, as described by the curvature of their Lagrangian configuration manifold. We focus on the case of spherical systems, which are integrable in the collisionless (mean field) limit despite the apparent persistence of local instability of trajectories even as $N \rightarrow \infty$. It is shown that when the singularity in the potential is removed, a null scalar curvature is associated with an effective, averaged, equation of state describing dynamically relaxed equilibria with marginally stable trajectories. The associated configurations are quite similar to those of observed elliptical galaxies and simulated cosmological halos. This is the case because a system starting far from equilibrium finally settles in a state which is integrable when unperturbed, but where it can most efficiently wash out perturbations. We explicitly test this interpretation by means of direct simulations.

astro-ph.CO↗

arXiv2005

Two Body Relaxation in Simulated Cosmological Haloes

This paper aims at quantifying discreetness effects, born of finite particle number, on the dynamics of dark matter haloes forming in the context of cosmological simulations. By generalising the standard calculation of two body relaxation to the case when the size and mass distribution are variable, and parametrising the time evolution using established empirical relations, we find that the dynamics of a million particle halo is noise-dominated within the inner percent of the final virial radius. Far larger particle numbers (~ 10^8) are required for the RMS perturbations to the velocity to drop to the 10 % level there. The radial scaling of the relaxation time is simple and strong: t_relax ~ r^2, implying that numbers >> 10^8 are required to faithfully model the very inner regions; artificial relaxation may thus constitute an important factor, contributing to the contradictory claims concerning the persistence of a power law density cusp to the very centre. The cores of substructure haloes can be many relaxation times old. Since relaxation first causes their expansion before recontraction occurs, it may render them either more difficult or easier to disrupt, depending on their orbital parameters. It may thus modify the characteristics of the subhalo distribution and effects of interactions with the parent. We derive simple closed form formulas for the characteristic relaxation times, as well as for the weak N-scaling reported by Diemand et al. when the main contribution comes from relaxing subhaloes (abridged).

astro-ph↗

arXiv2005

The persistence of the universal halo profiles

Simple simulations suggest that the phase space structure of haloes identified in cosmological calculations is invariant under the dynamics induced by sinking substructure satellites -- the background expands so as to leave the total distribution unchanged. We use a Fokker-Planck formulation to show that this is the case. The solutions obtained are valid for any mass spectrum of substructure, because the governing equation is linear in their mass weighed phase space distribution. If the clumps are considered solid, an exchange of poulations takes the form of an exponential instability, with characteristic timescale of the order of the dynamical friction time, leading to a low energy cutoff in the distribution function of the background and a constant density core. We show that there are long lived solutions with such a cutoff. They approximate a situation whereas the clumps are made of dense baryonic material. When stripping is important, as in the case of dissipationless substructure, it is likely that this situation is reversed -- the cutoff is now in the clump distribution function. A simple description suggests that this renders equilibria even more long lived. The results of this paper suggest that halo profiles similar to those found in dissipationless cosmological simulations are approximately invariant under the interaction induced by the presence of substructure satellites -- a necessary condition for the observed `universality'. In addition, the total profile, including baryons, should also be invariant; provided the latter are initially in the form of dense clumps, whose distribution follows that of the dark matter.

astro-ph↗

arXiv2004

Flat-Cored Dark Matter in Cuspy Clusters of Galaxies

Sand, Treu, & Ellis (2002) have measured the central density profile of cluster MS2137-23 with gravitational lensing and velocity dispersion and removed the stellar contribution with a reasonable M/L. The resulting dark matter distribution within r<50 kpc was fitted by a density cusp of r^{-beta} with beta=0.35. This stands in an apparent contradiction to the CDM prediction of beta~1, and the disagreement worsens if adiabatic compression of the dark matter by the infalling baryons is considered. Following El-Zant, Shlosman & Hoffman (2001), we argue that dynamical friction acting on galaxies moving within the dark matter background counters the effect of adiabatic compression by transfering the orbital energy of galaxies to the dark matter, thus heating up and softening the central density cusp. Using N-body simulations of massive solid clumps moving in clusters we show that indeed the inner dark matter distribution flattens (with beta approx 0.35 for a cluster like MS2137-23) when the galaxies spiral inward. We find as a robust result that while the dark matter distribution becomes core-like, the overall mass distribution preserves its cuspy nature, in agreement with X-ray and lensing observations of clusters.

astro-ph↗

arXiv2004

Dynamical-Friction Galaxy-Gas Coupling and Cluster Cooling Flows

We revisit the notion that galaxy motions can efficiently heat intergalactic gas in the central regions of clusters through dynamical friction. For plausible values of the galaxy mass-to-light ratio, the heating rate is comparable to the cooling rate due to X-ray emission. Heating occurs only for supersonic galaxy motions, so the mechanism is self-regulating: it becomes efficient only when the gas sound speed is smaller than the galaxy velocity dispersion. We illustrate with the Perseus cluster, assuming a stellar mass-to-light ratio for galaxies in the very central region with the dark-matter contribution becoming comparable to this at some radius $r_s$. For $r_s \la 400 {\rm kpc} \sim 3 r_{\rm cool}$--corresponding to an average mass-to-light ratio of $\sim10$ inside that radius--the dynamical-friction coupling is strong enough to provide the required rate of gas heating. The measured sound speed is smaller than the galaxy velocity dispersion, as required by this mechanism. With this smaller gas temperature and the observed distribution of galaxies and gas, the energy reservoir in galactic motions is sufficient to sustain the required heating rate for the lifetime of the cluster. The galaxies also lose a smaller amount of energy through dynamical friction to the dark matter implying that non--cooling-flow clusters should have flat-cored dark-matter density distributions.

astro-ph↗